doc: sending_patches.rst: Incorporate the old "Patches" wiki content

doc: sending_patches.rst: Incorporate the old "Patches" wiki content

Import as-is much of the old "Patches" wiki page to the current
sending_patches.rst file. This means we need to move patman to being
included in the higher level ToC and add a reference for "Custodians" in
the process document. A very minimal amount of content changing and
rewording is done here as part of the import, in order to make the
conversion easier.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>